91 S10 2WD 4.3L 173000mls

Hopefully someone can help me diagnose this problem. When the truck is cold it runs fine. However, after it warms up, during idle it revs up and down. It will eventually die out if I don't give it some gas. It will likely die out when I use the heater or turn on the headlights, most likely because it does not maintain the proper idle speed. I replaced the map sensor and the oxygen sensor with no avail. I just got the truck not to long ago so I'm not familiar with common problems with this truck. If anyone can help I thank you sooo much.

P.S. If the check engine light does not turn, will it do any good to run for codes?

Codes is the first thing to check for.
If it has any post back code no.
Look it over for vacuum leaks and sticking egr valve and clean TBI and and IAC valve.
How is the basic tune up?
Have you checked the base timing?
Fuel pressure?